How do I recognise too much humidity in the building?

Excessive humidity in buildings leads to increased mould formation and thus ultimately to impairment of the building fabric. In addition, the health of the occupants can also be considerably impaired.

Excessive humidity should therefore be avoided at all costs. However, in order to deal with the problems surrounding this topic in more detail and thus to be able to give tips on how to avoid too high air humidity, we must first examine the term "too high air humidity" in more detail. What exactly does too high mean? Opinions differ on this very point. This is also testified to by the fact that various studies on this topic (almost) all come to a different conclusion. We pass on our experience from our years of work here and do not want to deny anyone their opinion or their results.


Excessive humidity is essentially dependent on two factors: the amount of water in the air and the temperature. Warm air can absorb more water, and thus moisture, than cold air. This is because the air molecules "slide" further apart when it is warm and, simply put, more "water" fits into the spaces between the molecules. Now, the moisture "dissolved" in the air is not really a problem in the first place - but the water that condenses out of the air due to temperature changes. Or the level of this moisture and its (possibly negative) effect on various things. So when does the moisture stored in the air leave its host? If warm air with a lot of moisture cools down, the relative humidity rises at first. At some point, however, the point is reached where the maximum possible amount of water has been absorbed. If the air cools down further, water precipitates out of the air in the form of condensate.

This water is then (usually) the real problem for us and our house. So if we have a certain temperature and humidity in the room (e.g. 21° C and 45 % relative humidity), a cool surface can cool the air. This can be a surface of a building component, e.g. a wall or a window, which is cooler than the rest. As a result, the relative humidity rises at this point. Often into ranges far above 80 % or even to the point where we can see condensation (i.e. water). If a window becomes so cold that water condenses out of the room air (usually at the edges of the window), this is in most cases a sign of too high humidity. Excessive humidity in a building or flat therefore depends on the coldest spot in the room. In order to be able to make precise statements about the exact value, one would now have to know the temperature of the coldest surface in the room and calculate back what relative humidity must not be exceeded at a certain temperature.

There are also so-called hygrometers that display the relative humidity more or less accurately. The simplest models are analogue indicators with a horsehair or plastic hair connected to an indicator needle. These devices are quite common and cost from about 5 € in DIY stores. More expensive devices with a digital display and a real sensor cost between 15 and 20 €. Unfortunately, you cannot tell from the price of the devices how good they really are. Normally, tolerances of approx. 5 - 10 % of the displayed values are to be expected. However, devices with indications such as "too low", "good" or "too high" are usually to be regarded critically and often have no significance. In autumn and winter, normal values are between approx. 40 % and 45 % relative humidity at about 21° C room temperature. In summer, sometimes significantly higher.


When we dry laundry, cook a lot, shower or take a long bath, the relative humidity can quickly rise to over 60-70%. If we sleep with the window closed at night, 75 % relative humidity is not uncommon. A good way to detect excessive humidity without a measuring device is to open a window or door with a glass insert, in addition to visible condensation on window panes (in which case the humidity is clearly too high). Especially in winter, when it is cooler, you can orientate yourself by how much or how long a window pane fogs up from the outside when you open the window or door. As soon as you open a window (in winter), the outer glass pane fogs up to a greater or lesser extent. You can see this particularly well on a patio door. Condensation is usually particularly strong in the upper area of the window or door. The humidity in the room is higher the more pronounced the condensation is or the longer the window pane remains fogged up. If the entire window pane is slightly white or if even small water droplets form on the surface when you run your finger over it, you can assume that the air humidity in the room is very high. If the window remains misted up for significantly longer than two minutes, this is also a good indication of high humidity. If the window, the window frame or door locks/handles or hinges mist up from the inside without the window or door being opened, there is almost certainly too much humidity in the air and it should be ventilated urgently.



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